The field of the invention is that of integrated circuit processing with a process that includes a trench device isolation.
In modern silicon integrated circuit processing, space requirements have resulted in great popularity for device isolation by etching trenches and filling them with oxide, rather than the LOCOS isolation popular in the past.
The technology for etching and filling such isolation trenches is well advanced, though improvements are continuously being developed.
A popular method of trench fill includes a step of growing a thin layer of thermal oxide to passivate the trench sidewalls, before the main filling step. A drawback of such an approach on SOI (Silicon On Insulator) wafers is that the thermal oxide does not grow on the trench bottom and also grows nonuniformily along the trench sidewalls. The thermal oxide liner is thicker toward the center and top, leaving a negative angle or recess near the bottom corner of the trench, denoted by numeral 114 in FIG. 2. Such a recess leaves a void in the bottom corners when the main fill is done with HDP (high density plasma) oxide deposition.
Filling the entire trench with LPCVD oxide fills the negative angle at the bottom corners preventing corner void formation. However, the drawback of this approach is the formation of a seam or center void. Thus, voids remain a problem that is present in contemporary approaches to trench isolation. Any void present in the trench fill can become a receptacle during subsequent processing for conductive material such as polysilicon and may degrade product yield.
The invention relates to a trench fill process for trench isolation that produces void-free trench filling.
A feature of the invention is the filling of sidewall recesses left by the initial thermal oxide step.
Another feature of the invention is a thin filling layer of low density oxide fill having intermediate conformality: i.e. is thicker where it fills the bottom corners.
Another feature of the invention is filling the remaining aperture with HDP oxide.
Yet another feature of the invention is densifying the filling layer so that the final composite oxide fill has substantially the same etch rate for all three components.